The tooth geometry of bevel gears is always symmetrical to an axis of symmetry that runs radially through the center of the gear through the tooth. This is justified on the basis that gears are generally designed and produced to operate in both directions (forward and backward). Current production methods are limited in the degree of freedom for production and are optimized with the aim of producing many teeth of a toothed gear simultaneously. Depending on the direction of operation, an individual tooth during the transmission of torque has a load flank that is engaged and transmits the torque onto the load flank of the opposing tooth. When the toothed gear rotates, the nonworking flank of a tooth comes into contact either partially or not at all with the following opposing tooth of the meshing gear tooth as long as there is no reversal in the direction of rotation.
Toothed gears are generally designed for both directions of rotation irrespective of actual use. In practice, bevel-gear transmissions often do not require any reversal in the direction of rotation. Nevertheless, the flanks of the individual teeth of a toothed gear are produced and further machined for both rotational directions.
What is not considered here is the fact that when rotating in the reverse direction the nonworking flanks do not necessarily have the same function as the load flanks of their respective teeth. During the manufacturing of the toothed gear, the same amount of material is normally removed on the nonworking flank as on the load flank. Both tooth flanks subsequently undergo finishing to an equal extent, which is often costly, i.e., the nonworking flanks are completely hardened or receive surface treatment.